Conventionally, a gypsum-based building material such as a gypsum board, a reinforced gypsum board, a normal hard gypsum board, a glass mat gypsum board, a glass-fiber-nonwoven-fabric-containing gypsum plate, or a slug gypsum plate has an excellent performance in a fireproof property or fire resistance, a sound insulation property, a construction property, an economical property, and the like, and hence, has been used widely.
Such a gypsum-based building material is usually manufactured by adding water, etc., to, and mixing by a mixer, a gypsum composition wherein a calcined gypsum and a variety of additives are mixed preliminarily, to provide a gypsum slurry (plaster slurry), and shaping into a predetermined shape, subsequently drying, and cutting, such a gypsum slurry and a base paper for board, a glass mat, a glass fiber nonwoven fabric or the like.
A gypsum-based building material is such that its lightweight property depends on an amount of gypsum and an amount of a bubble or void in a gypsum hardened body that is mainly used as a core material. For this reason, an amount of gypsum is decreased, that is, an amount or proportion of an occupying bubble or void is increased, so that it is possible to reduce a total specific gravity of, and attain a light weight of, a gypsum-based building material.
However, a physical strength of a gypsum hardened body that composes a gypsum-based building material is reduced as its specific gravity is reduced. For this reason, with respect to a gypsum board wherein a gypsum hardened body is a core material and a base paper for board is used as a surface material, a gypsum plate wherein a gypsum hardened body is a core material and a glass mat is used as a surface material, or a gypsum plate wherein a gypsum hardened body is a core material and a glass fiber nonwoven fabric (glass tissue) is embedded in a surface thereof, a strength of a gypsum board or gypsum plate is also reduced as a specific gravity of a gypsum hardened body that is a core material is reduced.
Patent Document 1 discloses, as an example, a gypsum hardened body with a low specific gravity that is provided by increasing and homogenizing diameters of added bubbles to provide good spherical shapes when bubbles are added to a gypsum slurry wherein a gypsum composition and water are mixed.
However, a strength of a gypsum hardened body could not have been sufficiently high, even by a method disclosed in Patent Document 1.
Furthermore, compounding a starch has been studied for a purpose of improving a strength of a gypsum hardened body. In particular, Patent Document 2 discloses, as an example, that a strength of a gypsum hardened body with a low specific gravity is drastically improved by using a pregelatinized starch.
However, as a pregelatinized starch is used, a state of bubbles added for fabricating a gypsum hardened body with a low specific gravity is such that a bubble with a large diameter and a bubble with a small one are mixed and a bubble with a large diameter is in a deformative state. Such a deformative state of a bubble has been known as, for example, a precursory phenomenon for partially releasing of a gypsum hardened body and a base paper for board on a surface thereof when a gypsum board is provided, and in such a case, there is a problem in that blistering is caused on a surface of a gypsum board.
Furthermore, as a pregelatinized starch is compounded, an amount of water needed for mixing a gypsum composition with water is drastically increased, so that there is also a problem in that cost for drying of a gypsum hardened body is increased.
Moreover, even if an amount of added pregelatinized starch is increased, an effect of strength improvement is caused to plateau when such an amount for addition is a certain amount or more, so that there is also a problem in that it is not possible to sufficiently deal with an application that requires, in particular, attainment of both a light weight and a strength.